In a hydrometallurgical zinc production process, a zinc-containing ore is concentrated, roasted and leached into sulphuric acid. At least some of the zinc concentrate can also be leached directly without roasting. In addition to zinc, copper, cobalt, nickel and cadmium are also released during leaching. Traditionally these metals or impurities are removed or separated from the concentrated zinc sulphate solution by reduction with zinc powder in the solution purification process. In addition to zinc powder arsenic or antimony trioxide is utilized as a reagent to precipitate these metals out from the solution. The separation of metals can be carried out in one or several stages in precipitation reactors or the equivalent. After removal of the above-mentioned metals, zinc is reduced from the solution electrolytically. Impurities must be removed from the zinc sulphate solution in the solution purification stages, so that successful and effective electrolysis can be achieved to reduce the zinc. The zinc raw material may also include chloride and this has to be removed from the zinc sulphate solution to a certain level before electrolysis, where it would create problems of corrosion and work hygiene.
The separation of copper from the zinc sulphate solution is important, because copper is precipitated with zinc onto the cathode in electrolysis, causing impure zinc. In addition, copper may cause the generation of local couples, which reduces the current efficiency. Conventionally copper is cemented with zinc powder either in a separate copper removal stage or together with cobalt and cadmium. Cementation is an electrochemical exchange reaction, where the more precious metal is precipitated and the baser metal dissolves. When copper is precipitated in a separate stage, it is preferable to leave a small amount of copper in the solution for the following solution purification stages, because a small amount of copper is beneficial especially in the cobalt precipitation stage. The following reactions occur in the copper cementation stage:CuSO4+Zn→ZnSO4+Cu  (1)CuSO4+Cu+H2O→+Cu2O+H2SO4  (2)2CuSO4+Zn+H2O→+Cu2O+ZnSO4+H2SO4  (3)
The copper precipitate can be used as raw material in a copper smelter for example.
The second stage of solution purification is generally the separation of cobalt and nickel. Zinc powder is often used in this stage in addition to arsenic and antimony compounds as extra reagents. Nickel and cobalt precipitation, however, require the presence of copper in the solution. The final purification stage is usually the removal of cadmium.
As mentioned above, the amount of chloride in the zinc sulphate solution has to be reduced to a certain level (in general to less than 200 mg/l) before the solution is routed to electrolysis. When the zinc sulphate solution is prepared from the concentrate, which is first roasted, most of the chlorine is already removed during roasting. The aim is to make a concentrate mixture from various kinds of concentrate where the Cl content of the solution generated does not rise too high. Since direct leaching processes without roasting have become so widespread, Cl removal has been and will continue to be an essential process stage.
The amount of zinc powder required in the solution purification of zinc sulphate solution is from 2-10% of the process output, depending on the plant. Thus the zinc consumed must be produced again in electrolysis, so that the demand for electricity is increased considerably. The amount of zinc produced is also correspondingly smaller.
Since the costs of preparing zinc a second time are considerable, various methods of reducing the amount of zinc powder used in cementation have been sought. EP patent application 134644 suggests that some kind of flocculant be introduced into the solution to improve the separation level of metal impurities and reduce the amount of zinc powder. Redox-potential regulation has also been used in an attempt to control the feed of a suitable amount of zinc powder, which is mentioned in U.S. Pat. No. 4,252,622.
AU patent 536376 mentions that in addition to zinc powder cementation, copper can be removed from a zinc sulphate solution by cementation with metallic iron, steel, sulphide precipitation or by precipitation as an alkaline sulphate, as well as liquid-liquid extraction and ion exchange. Apart from this sentence however, the patent does not describe any other method than zinc powder cementation.
In Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, VCH Verlagsgesellschaft & Co, Germany 1996, Vol. 28A, it is mentioned in the section on Zinc by Graf, G., p. 524, that the removal of metals more precious metals than zinc from a zinc sulphate solution using ion exchangers is still at the development stage.
U.S. Pat. No. 4,005,174 describes the removal of chloride from a zinc sulphate solution. In this method, monovalent copper is introduced into a zinc sulphate solution containing chloride that forms copper chloride at a pH of below 2.6, which then precipitates out of the solution. Monovalent copper is obtained in the solution by adding for instance copper(II)sulphate and zinc powder to the solution. Copper powder can be used in place of zinc powder. One alternative is to feed monovalent copper oxide Cu2O into the solution. When copper (I) oxide is fed directly into the solution, zinc powder is saved from the feed for the chloride removal stage. In this case, Cu2O is produced from the copper chloride generated in chloride removal e.g. using sodium hydroxide. After chloride removal, 0.5 g/l of divalent copper should remain in the solution, which is reduced using zinc powder to monovalent. The solution from Cl removal is routed to the copper removal stage and the soluble (divalent) copper generated during Cl removal is precipitated using zinc powder precipitation. For instance, the method is used in the handling of galvanization dusts and copper is not normally present in these processes.